Types of heat transfer in physics

The branch of physics that studies heat transfer and its types is based on the mechanical theory of heat put forward in the 18th century by the great Russian scientist  Lomonosov. Modern researchers are actively using this process in various spheres of human life, for example, in technology, aviation and rocketry.

Description of the Heat Transfer Process

Heat transfer is one of the most important physical processes, consisting of several simple transformations. During it, heat is transferred from one object to another or inside the body in the presence of a temperature difference. Heat energy is present in the following media:

• gases;
• liquids;
• solid bodies.

Heat transfer methods examples

Heat transfer is a spontaneous process that takes place in free space. Energy spreads from objects that have a high temperature to bodies with a lower index. Research by scientists suggests that heat transfer is too complex to be considered as a single process. In this regard, the physical phenomenon was divided into the following three types:

• thermal conductivity;
• convection;
• radiation.

Thermal conductivity characteristic

Thermal conductivity is the transfer of energy from object to object or from one part of a certain physical body to another through the thermal motion of molecules and atoms. It should be noted that during this phenomenon, matter does not move, only internal energy is transferred. The following experiment allows observing thermal conductivity:

• Several nails are attached to a metal rod on wax.
• One end of the rod is firmly fixed in the tripod, and the other begins to be heated.

After a while, the nails fall off in turn.

This is due to the melting of the wax, which causes the temperature of the metal to rise. The fact that the nails did not fall off at the same time indicates a gradual heating of the rod. Consequently, the internal energy of the body, as it increased, was transferred from the hot end to the cold one.

Heat transfer has another explanation based on the internal structure of matter. The particles of the heated end of the rod increase their energy due to external influence. As a result, their oscillation becomes more intense, due to which a part of the obtained potential is transferred by the molecules to neighboring particles, which also begin to oscillate faster.

The process of transferring energy gradually covers the entire core. The result of its increase is an increase in the temperature of the object.

Gaseous substances have an even lower thermal conductivity. You can prove the statement empirically:

• A test tube containing air is fixed in a stand.
• A lighted spirit lamp is placed under it.
• If you put your finger in the test tube, you will not feel the heat. The experiment allows us to conclude that air, like other gases, poorly transfers internal energy.

Metallic bodies are considered the best conductors of heat, and highly rarefied gases are the worst. The reason for this is their molecular structure. Particles of gaseous substances are located at large distances from each other, and therefore rarely collide, which is why the transfer of heat is much slower than in solids. In terms of thermal conductivity, liquids are located between gases and solid objects.

Description of convection of the Heat Transfer Process

Convection is another way of transferring heat. Its essence lies in the transfer of internal energy by layers of liquid or gaseous substances.

Since convection occurs only when substances move, such a process can only be carried out in liquids and gases. It is known that physical bodies in these two states conduct heat poorly, but thanks to concentration they can still be heated.

The effective application of this process can be observed in the cold season, when the air is warmed up in rooms equipped with steam heating batteries. This type of heat transfer can be observed in a simple experiment:

• A potassium permanganate crystal is carefully lowered to the bottom of a flask filled with water.
• The container is heated in the place where the manganese acid salt lies.
• After a while, colored streams of water begin to rise from the bottom.
• Having risen to the upper layers, the jets descend.

The lower layer of liquid expands when heated, which leads to an increase in its volume and a decrease in density. Under the influence of the Archimedean force, the heated part of the substance moves higher. A cold liquid is lowered onto the vacant place, which rises as it warms up. In this case, the internal energy is transferred by the streams of water moving upwards.

Heat transfer occurs in a similar way in gases. So, if a paper turntable is placed over a heat source, then it begins to rotate. The blades of the object are set in motion because the least dense layers of heated air rise due to the effect of buoyancy on them, at the same time the cold layers descend, taking the place of the warm ones. This movement of air makes the turntable spin.

Definition of radiation

The last form of heat transfer is radiation. You can feel it by bringing your hand to a switched on electric light bulb, a heating battery, a spiral of a heated electric stove, a hot iron, etc. Experimentally, you can identify radiation as follows:

• The metal heat sink, which has shiny and black surfaces, is fixed in a tripod.
• A pressure gauge is attached to it.
• Boiling water is poured into a vessel, one side of which is painted white and the other black.
• The container with water is turned to the black surface of the heat sink, first with the white and then with the black side.
• In both cases, the water level in the knee of the manometer decreases.
• But it should be noted that when the black side of the vessel is facing the heat sink, there is less liquid in the elbow.

The change in the water level in the manometer is explained by the fact that the air in the heat receiver begins to expand. But the expansion of the gas is possible only when heated, which means that the substance received energy from the container with boiling water. It is known that air has poor thermal conductivity, but there is no convection in this situation, since the vessel is located at the same level with the heat sink, therefore, the container emits thermal energy.